Unusual Equilibria Involving Group 4 Amides, Silyl Complexes, and Silyl Anions via Ligand Exchange Reactions

Silyl anion SiButPh2- (2) was found to substitute an amide ligand in Zr(NMe2)4 (3) to give the disilyl complex Zr(NMe2)3(SiButPh2)2- (1a) and Zr(NMe2)5- (1b) in THF. The reaction is reversible, and nucleophilic amide NMe2- attacks the Zr−SiButPh2 bonds in 1a or Zr(NMe2)3(SiButPh2) in the reverse reaction, leading to an unusual ligand exchange equilibrium 2 3 + 2 2 ⇌ 1a + 1b (eq 1). The silyl anion 2 selectively attacks the −N(SiMe3)2 ligand in Zr(NMe2)3[N(SiMe3)2] (6) to give 1a and N(SiMe3)2- (7). Reversible reaction occurs as well, where 7 selectively substitutes the silyl ligand in Zr(NMe2)3(SiButPh2)2- (1a) or Zr(NMe2)3(SiButPh2), giving the equilibrium 6 + 2 2 ⇌ 1a + 7 (eq 3). The thermodynamics of these equilibria has been studied:  For eq 1, ΔH° = −8.3(0.2) kcal/mol, ΔS° = −23.3(0.9) eu, and ΔG°298K = −1.4(0.5) kcal/mol at 298 K; for eq 3, ΔH° = −1.61(0.12) kcal/mol, ΔS° = −2.6(0.5) eu, and ΔG°298K = −0.8(0.3) kcal/mol. In both equilibria, the enthalpy changes for the forward reactions outweigh the entropy changes, and therefore the substitutions of amide ligands in Zr(NMe2)4 (3) and Zr(NMe2)3[N(SiMe3)2] (6) to afford the disilyl complex 1a are thermodynamically favored. The following equilibria were also observed and studied:  Zr(NMe2)3[N(SiMe3)2] (6) + Si(SiMe3)3- (9) ⇌ Zr(NMe2)3[Si(SiMe3)3] (10) + N(SiMe3)2- (7) and Zr(NMe2)4 (3) + 9 ⇌ 10 + Zr(NMe2)5- (1b).